1. Field of the Invention
This invention relates generally to the fabrication of a giant magnetoresistive (GMR) or a tunneling magnetic junction (TMJ) magnetic field sensor in the current-perpendicular-to-plane (CPP) configuration, more specifically to self-stabilization provided by a vortex configuration of the magnetization of the free layer of such a sensor.
2. Description of the Related Art
The CPP (current perpendicular to plane) configuration of a giant magnetoresistive (GMR) sensor or tunneling magnetic junction (TMJ) sensor is a multi-layered configuration of magnetic and non-magnetic layers which includes a magnetically free layer, whose magnetic moment is free to respond to outside magnetic stimuli, separated by a non-magnetic layer from a magnetically “pinned” layer, whose magnetic moment is fixed in direction. In the GMR sensor the non-magnetic layer is an electrically conducting, non-magnetic layer. In the TMJ sensor the non-magnetic layer is a tunnel barrier layer formed of insulating material. In the GMR sensor, the motion of the free layer magnetic moment relative to the pinned layer magnetic moment changes the resistance of the sensor so that a “sense” current passing perpendicularly through the layers produces measurable voltage variations across the sensor. In the TMJ sensor, the motion of the free layer magnetic moment relative to the pinned layer magnetic moment affects the availability of tunneling states across the tunnel barrier layer and thereby changes the amount of perpendicular current that passes through the layer.
One of the problems associated with the design of a CPP sensor in either the GMR or MTJ configuration is the stabilization of its magnetically free layer. In general, the magnetization of the free layer would be fragmented into a multiplicity of domains. Small domains at the lateral edges of the free layer have magnetization directions that are relatively easy to move. If no edge domains are formed, the magnetization at the free layer edges are referred to as “uncompensated poles,” because there are no adjacent magnetization vectors to hold them in a fixed position. Because of these uncompensated poles or edge domains, the general magnetization of the free layer is not fixed in time, but changes easily as a result of thermal agitation and/or external fields, leading to a form of noise in the sensor signal called Barkhausen noise. It is, therefore, desirable to maintain the free layer in a single domain state, which would be much less subject to agitation. This process is called stabilization. Since the magnetization of the free layer is typically directed “longitudinally” (within the plane of the layer and within the plane of the air-bearing-surface (ABS) of the sensor), whereas the magnetization of the pinned layer is typically directed transversely to the ABS plane, the stabilization of the free layer is accomplished by what is called a “longitudinal bias layer (LBL) (or biasing layer).”. These layers are generally permanent magnets (called “hard bias”), which couple magnetostatically to uncompensated magnetization vectors at the lateral edges of the free layer and, in effect, hold them in position and produce the single domain. These biasing layers may be placed at the stack edges and insulated from them, or placed “in-stack.”
In the CPP configuration, placing conducting permanent magnets on each side of the layer configuration produces an even more severe and immediate problem, since the magnets would give the sense current a pathway that bypasses the sensor configuration. Although this problem can be ameliorated by placing insulating layers between the biasing layers and the sensor, this solution increases the difficulties of fabricating the sensor.
It is seen from the above brief discussion, that maintaining the domain stabilization of the free layer is a problem of concern and methods adopted to provide such stabilization are not completely satisfactory. A completely different approach to stabilizing the magnetization of a free layer is to magnetize it in such a way that uncompensated edge magnetization does not exist, so that biasing at the edges is no longer required. The approach of the present invention is to magnetize the free layer in a vortex configuration, so that its magnetization vectors are directed circumferentially about an axis passing vertically through the layer planes of the GMR device. As is well known, such a magnetization direction occurs about a long current carrying wire, so it is to be expected that the use of a unidirectional current will be important in establishing such a vortex configuration within the free layer plane. The vortex configuration is not unknown in the prior art and it has been discussed in relationship to several inventions relating to MRAM configurations rather than sensor configurations.
Monsma et al. (U.S. Pat. No. 6,269,018) teaches a magnetic tunneling junction MRAM element in which the cell is formed in the shape of a disk and the magnetic fields in the ferromagnetic layers are vortex-shaped. The state of the device is written by changing the handedness of the vortex. It is also noted that the vortex shaped field reduces unwanted interactions between neighboring cells.
Engel et al. (U.S. Pat. No. 6,654,278) teaches the formation of an MRAM device in which a reference layer has a vortex-shaped magnetic field with a zero net magnetic moment. During read operations, a magnetic field shifts the center of the vortex, causing a net magnetic moment to be acquired by the reference layer and, thereby, enabling the magnetic moment of the free layer to be determined. Thus the reference layer plays an active role in the read operation of the cell.
Abraham et al. (U.S. Pat. No. 6,590,750) teaches the formation of a magnetic tunneling junction in which tunneling is restricted to specific areas between the ferromagnetic layers in which the magnetic moments can be reliably switched. It is noted that the curling of the magnetic field lines is a cause of the unreliability in the conventional tunneling junction cell.
The present invention provides a vortex shaped magnetic field in the free layer of a magnetic tunneling junction read head sensor. The vortex shaped field is self stabilized and requires no longitudinal bias layers, yet responds effectively to external magnetic field variations by a shift of its center. Among its advantages, the sensor so formed permits the formation of a more effective shield configuration that reduces unwanted side reading.